Release
Use the Release node to release particles into the model from a selected set of domains. The release times, initial position, and initial value of any auxiliary dependent variables can be specified. The sections available are based on the Formulation in the physics interface Particle Release and Propagation section. If Newtonian, Newtonian, first order, Lagrangian, or Hamiltonian are selected, also enter an initial velocity.
Release Times
Select a Distribution function: List of values (default), Uniform, Normal., or Lognormal.
List of Values
Enter Release times (SI unit: s) or click the Range button () to select and define a range of specific times. At each release time, particles are released with initial position and velocity as defined in the following sections.
Uniform
Enter the Number of values, along with the First time value (SI unit: s) and the Last time value (SI unit: s). In addition, select whether the Sampling from distribution should be Deterministic or Random. When Deterministic is selected, an array of length Number of values of uniformly spaced release times is generated. This array starts with the First time value and ends with the Last time value exactly. The release times are reproducible each time the solution is computed.
When Random is selected, an array of random numbers of length Number of values, with a minimum lower limit of the First time value and maximum upper limit of the Last time value is generated. In this case, a release time of exactly the first and last time values is extremely unlikely.
Normal and Lognormal
Enter the Number of values along with the Mean (SI unit: s) and the Standard deviation (SI unit: s). In addition, select whether the Sampling from distribution should be Deterministic or Random. When Deterministic is selected, an ordered array of length Number of values of normally distributed release times is generated. The release times are more closely spaced around the value entered for the Mean and the spacing of the release time drops off according to the value of the Standard deviation. The release times are reproducible each time the solution is computed.
When Random is selected, the normal or lognormal distribution is generated by pseudorandom sampling, so there may be some statistical error.
For models using The Charged Particle Tracing Interface the Release Current Magnitude section is available instead of Release Times when the Particle release specification is set to Specify current. Enter a value for the Release current magnitude I (SI unit: A). The default is 1 nA. For models using The Particle Tracing for Fluid Flow Interface the Mass Flow Rate section is available instead of Release Times when the Particle release specification is set to Specify mass flow rate. Enter a value for the Mass flow rate (SI unit kg/s). The default is 1dn either case, all particles are released at time = 0. The expression for the Release current magnitude or Mass flow rate should only depend on parameters and global variables, not on particle variables such as the particle mass.
Initial Position
Select an Initial position: Mesh based (the default), Density, or Random.
Mesh Based
For Mesh based the particles are released from a set of positions determined by a selection of geometric entities (of arbitrary dimension) in the mesh.
Select a Refinement factor between 1 and 5 (the default is 1), where the centers of the refined mesh elements are used. Thus, the number of positions per mesh element is refine^dim, except for pyramids, where it is (4*refine2-1)*refine/3.
Density
For Density the particles are positioned in the selected domains by sampling from a user-defined spatial distribution. Enter a value for the Number of particles per release N (dimensionless). The default is 1. Enter a value or expression for the Density proportional to ρ (dimensionless). The default is 1.
The field Density proportional to is an expression; the resulting particle distribution approximately has a density that is proportional to this expression. The resulting distribution looks a bit random, and it depends on the underlying mesh.
Select a Release distribution accuracy order between 1 and 5 (the default is 5), which determines the integration order that is used when computing the number of particles to release within each mesh element. The higher the accuracy order, the more accurately particles will be distributed among the mesh elements.
The Position refinement factor (default 0) must be a nonnegative integer. When the refinement factor is 0, each particle is always assigned a unique position, but the density is taken as a uniform value over each mesh element. If the refinement factor is a positive integer, the distribution of particles within each mesh element is weighted according to the density, but it is possible for some particles to occupy the same initial position. Further increasing the Position refinement factor increases the number of evaluation points within each mesh element to reduce the probability of multiple particles occupying the same initial position.
If the model includes a Particle-Particle Interaction then there is a risk that the force may become infinite if the initial particle positions are not unique. When modeling particle-particle interactions the Position refinement factor should be 0 to ensure that initial positions are unique. However, it may then be necessary to refine the mesh to get an accurate initial distribution of particle positions if the density is not uniform.
Random
For Random the particles are released at random positions within the selected entities. If particles are released at multiple release times, the initial positions are uniquely generated for each release time. By contrast, for Density the particle positions are the same at each release time.
The particle positions are determined in the following way. First a random mesh element is selected for each particle with probability proportional to the element size, so that particles are more likely to be released from larger mesh elements than smaller elements. After the mesh element is selected, random local coordinates are chosen within the element, then converted to global coordinates. The particle is then placed at this location.
The Number of particles per release is the number before velocity, release time and auxiliary dependent variable multiplication, so the actual number of model particles generated by the Release feature may be significantly larger.
Initial Velocity
For The Mathematical Particle Tracing Interface this section is shown when Newtonian, Newtonian, first order, Lagrangian, or Hamiltonian is selected as the Formulation.
For The Charged Particle Tracing Interface and The Particle Tracing for Fluid Flow Interface this section is shown when Newtonian or Newtonian, first order is selected as the Formulation.
Select an option from the Initial velocity list: Expression (the default), Kinetic energy and direction, Constant speed, spherical, Constant speed, hemispherical, Constant speed, cone, Constant speed, Lambertian (3D only), or Maxwellian.
For Expression enter coordinates for the Initial particle velocity v0 (SI unit: m/s) based on space dimension. The defaults are 0 m/s.
When releasing particles from domains with The Particle Tracing for Fluid Flow Interface, enter a value or expression for the Velocity field u. If another physics interface that computes the velocity field is present, then this interface can be selected from the list. This allows the particles to be released with initial velocity equal to that of the surrounding fluid.
For Kinetic energy and direction enter the Initial kinetic energy E0 (SI unit: J). The default is 1 keV. Then enter coordinates for the Initial particle direction L0 (dimensionless) based on space dimension. The expression for the initial direction is automatically normalized to unity.
The option Kinetic energy and direction is not supported for The Particle Tracing for Fluid Flow Interface.
For Constant speed, spherical a total of Nvel particles are released in a circle or sphere at each release point, each with the same speed. Enter the following:
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Speed v (SI unit: m/s). The default is 1 m/s.
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Number of particles in velocity space Nvel (dimensionless). The default is 200.
For Constant speed, hemispherical a total of Nvel particles are released in a hemisphere or half circle at each release point, each with the same speed. Enter the:
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Speed v (SI unit: m/s). The default is 1 m/s.
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Number of particles in velocity space Nvel (dimensionless). The default is 200.
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A direction vector for the Hemisphere axis r (dimensionless). This sets the direction of the north pole of the hemisphere or half circle.
For Constant speed, cone a total of Nvel particles are released at each release point, each with the same speed. The velocity distribution is a cone of angle α for 3D components and an arc of half-angle α for 2D. Enter the:
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Speed v (SI unit: m/s). The default is 1 m/s.
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Number of particles in velocity space Nvel (dimensionless). The default is 200.
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A direction vector for the Cone axis r (dimensionless). This sets the direction of the cone axis for 3D components or the centerline of the arc for 2D.
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A Cone angle α (SI unit: rad).
The Constant speed, Lambertian option is only available in 3D. It generates Nvel particles for each release point, each with the same speed. The velocity distribution is a hemisphere in which the angle distribution follows the cosine law. Enter the:
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Speed v (SI unit: m/s). The default is 1 m/s.
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Number of particles in velocity space Nvel (dimensionless). The default is 200.
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A direction vector for the Hemisphere axis r (dimensionless). This sets the direction of the north pole of the hemisphere.
For Maxwellian enter the:
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Temperature T0 (SI unit: K). The default is 293.15 K.
For Constant speed, spherical, Constant speed, hemispherical, Constant speed, cone, Constant speed, Lambertian, and Maxwellian, select an option from the Sampling from distribution list: Deterministic (the default) or Random. If Deterministic is selected, the initial velocity is computed by sampling from the velocity distribution in a deterministic and reproducible manner. If Random is selected, the particle velocity is sampled from the distribution using pseudorandom numbers and may not always generate the same results.
For Constant speed, spherical, Constant speed, hemispherical, Constant speed, cone, and Constant speed, Lambertian, select the Allow nonuniform speeds in distribution check box to allow particles from the same release point to have different initial speeds. This check box has no effect on the initial velocity directions of the released particles, only their velocity magnitudes. If this check box is selected, expressions for the initial speed in terms of the particle index can return a unique value for each particle in the distribution.
For example, if Constant speed, hemispherical is selected from the Initial velocity list, the Allow nonuniform speeds in distribution check box is selected, and the expression for the initial speed is 2[m/s]*(random(pt.pidx)+0.5), then particles will be released with directions sampled from an isotropic hemisphere and with speeds sampled pseudorandomly between 0 and 2 m/s.
Released Particle Properties
Select an option from the Released particle properties list. At least one instance of the Particle Properties node is always available, because it is a default feature. If other instances of the Particle Properties feature have been added to the model, then they can be selected from the list. Use this input to specify which type of particle is released when modeling multiple particle species.
Initial Particle Temperature
This section is only available for The Particle Tracing for Fluid Flow Interface and is only shown when the Compute particle temperature check box is selected in the physics interface Additional Variables section. Enter a value or expression for the Initial particle temperature Tp,0 (SI unit: K). The default value is 293.15 K.
Initial Particle Mass
This section is only available for The Particle Tracing for Fluid Flow Interface and is only shown when Specify particle mass is selected from the Particle size distribution list in the physics interface Additional Variables section.
The initial particle mass can be a single value or can be sampled from a distribution. Select an option from the Distribution function list: None (the default), Normal, Lognormal, Uniform, or List of values.
When None is selected, enter an initial value mp,0. The default value is 109 kg.
Select Normal to create a normal distribution function, Lognormal to create a log-normal distribution function, or Uniform to create a uniform distribution function. For any of these distributions, select an option from the Sampling from distribution check box: Deterministic (the default) or Random. For Random sampling the mean and standard deviation may not be exactly equal to the specified values but will statistically converge as the number of particles is increased.
The Number of values N (default 1) sets the number of values that are sampled from the distribution function at each release point.
For the Normal or Lognormal distribution enter the Mean particle mass (default 109 kg) and Particle mass standard deviation (default 1010 kg). For the Uniform distribution enter the Minimum particle mass mp,min (default 109 kg) and the Maximum particle mass mp,max (default 2 × 109 kg). Select List of values to enter a list of numerical values for the initial particle mass directly.
Initial Particle Diameter
This section is only available for The Particle Tracing for Fluid Flow Interface and is only shown when Specify particle diameter is selected from the Particle size distribution list in the physics interface Additional Variables section. The options in this section allow you to release many particles from each release point with different initial diameters.
The initial particle diameter can be a single value or can be sampled from a distribution. Select an option from the Distribution function list: None (the default), Normal, Lognormal, Uniform, or List of values.
When None is selected, enter an initial value dp,0. The default value is μm.
Select Normal to create a normal distribution function, or Uniform to create a uniform distribution function. The Number of values N (default 1) sets the number of points in the distribution function. For the Normal distribution enter the Mean particle diameter (default μm) and Particle diameter standard deviation (default 0.1 μm). For the Uniform distribution enter the Minimum particle diameter dp,min (default μm) and the Maximum particle diameter dp,max (default μm).
Select Lognormal to create a log-normal distribution function. Then select one of the following options from the Specify list:
For Mean and standard deviation (the default), enter the following:
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Number of values N (dimensionless, default 1),
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Mean particle diameter μ (SI unit: kg, default 1 μm), and
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Particle diameter standard deviation σ (SI unit: m, default 0.1 μm).
For Count median diameter and geometric standard deviation, enter the following:
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Number of values N (dimensionless integer, default 1),
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Count median diameter CMD (SI unit: m, default 1 μm), and
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Geometric standard deviation GSD (dimensionless, default 1.1).
For Sauter mean diameter and geometric standard deviation, enter the following:
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Number of values N (dimensionless integer, default 1),
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Sauter mean diameter CMD (SI unit: m, default 1 μm), and
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Geometric standard deviation GSD (dimensionless; default 1.1).
For a Normal, Lognormal, or Uniform distribution, select an option from the Sampling from distribution check box: Deterministic (the default) or Random. For Random sampling the mean and standard deviation may not be exactly equal to the specified values but will statistically converge as the number of particles is increased.
Alternatively, select List of values to enter a list of numerical values for the initial particle diameter directly.
Initial Multiplication Factor
This section is only available for The Particle Tracing for Fluid Flow Interface and is only shown when the Enable macroparticles check box is selected in the physics interface Additional Variables section. Enter a value or expression for the Initial multiplication factor nn,0 (dimensionless). The default value is 1.
Initial Value of Auxiliary Dependent Variables
This section is available if an Auxiliary Dependent Variable has been added to the model.
For each of the Auxiliary Dependent Variables added to the model, choose a Distribution function for the initial value of the auxiliary dependent variables and whether the initial value of the auxiliary dependent variables should be a scalar value or a distribution function.
The number of particles simulated can increase substantially and the following options are available for each of the Auxiliary Dependent Variables added to the model.
When None is selected, enter an initial value (rp0).
For Normal, the initial values are sampled from a normal or Gaussian distribution. Enter the Number of values (default 1), Mean (default 0), and Standard deviation (default 1).
For Lognormal, the initial values are sampled so that their logarithms follow a normal distribution. Enter the Number of values (default 1), Mean (default 0), and Standard deviation (default 1). An advantage of the Lognormal distribution over the Normal distribution is that, if the Mean is positive, all the sampled values will be positive.
For Uniform, enter the Number of values (default 1), Minimum (default 0), and Maximum (default 1).
Select List of values to enter a set of numerical values directly.
For Normal, Lognormal, or Uniform, select an option from the Sampling from distribution list: Deterministic (the default) or Random. For Random the mean and standard deviation will statistically converge to the specified values as the Number of values is increased.
The Number of values sets the number of values that are sampled from the distribution function at each release point. So, if particles are released at 100 points, and the Number of values is 100, a total of 10,000 particles will be released. If a number of velocity values are also sampled at each release point, this number might be substantially higher.
By default auxiliary dependent variables are initialized after all other degrees of freedom. Select the Initialize before particle momentum check box to compute the initial value of the auxiliary dependent variable immediately after computing the initial particle position. By selecting this check box it is possible to define the initial particle velocity as a function of the auxiliary dependent variables. The order of initialization for the auxiliary dependent variables is described in more detail in Initialization of Auxiliary Dependent Variables in the Particle Tracing Modeling chapter.
Advanced Settings
This section is shown if Newtonian or Newtonian, first order is selected from the Formulation list in the physics interface Particle Release and Propagation section.
By default the Subtract moving frame velocity from initial particle velocity check box is cleared. Select this check box to offset the initial velocity of released particles if the frame is moving. If the check box is selected, choose an option from the Background velocity check box: From rotating frame (the default) or User defined. If From rotating frame is selected then there should be an active instance of the Rotating Frame feature in the model. If User defined is selected, enter values or expressions for the components of the background velocity vb (SI unit: m/s) directly. Thus, if this check box is cleared, particle velocity is always initialized with respect to the moving frame, which may be noninertial. If the check box is selected, the velocity is initialized with respect to the laboratory frame (that is, a fixed noninertial frame).